"Our only objective is to rid the country of the Thaksin regime," added Mr Suthep, a former deputy prime minister under the previous Democrat-led government.

Ms Yingluck has invoked special legislation which authorises her to deploy police and soldiers to counter security threats.

The protesters' actions "threaten the stability of the government," Ms Yingluck said in a brief televised address.

The protest group is partly made up of pro-establishment yellow shirts, previously known for takeovers of Thailand's airports and more but incorporates other new groups.

US calls for calm in Bangkok

Demonstrations in the last few days have been the largest since 2010, when the current government's red shirt supporters locked down the city centre and were only moved on in a bloody crackdown by the military.

The US has called for calm in Thailand, with State Department spokeswoman Jen Psaki saying that Washington is monitoring to situation closely.

"We urge all sides to refrain from violence, exercise restraint, and respect the rule of law," she said.

"Violence and the seizure of public or private property are not acceptable means of resolving political differences."

The US official also called on the Thai government to uphold freedom of press and guarantee the safety of journalists.